One of the major goals when collecting and condensing radiation, particularly visible light, from a source into a target is the maximization of the brightness of the light at the target. Various configurations using on-axis elliptical and parabolic reflectors, and off-axis reflectors of various shapes have been used. Since the brightness of the image created at the target theoretically only can be conserved in an ideal optical system (and is reduced in a non-ideal system) it is impossible to increase the total flux at the target above the amount which is emitted by the source.
One common technique used to combat this fundamental limitation is the use of an arc lamp as the source in combination with a retro-reflector such that the light emitted from one side of the arc lamp is redirected by the retro-reflector back through the arc. Since the absorption of the reflected light by the arc is very small, light emitted from the opposite side of the arc lamp comprises both the light radiating from the arc itself as well as the retro-reflected light. Thus, the total light flux emitted from the side of the lamp opposite the retro-reflector is effectively doubled. Other prior art methods have extended this concept by reflecting light from the arc back into itself multiple times, thus increasing the flux further as in U.S. Pat. No. 4,957,759 to Goldenberg et al.
U.S. Pat. No. 5,707,131, the specification of which is herein incorporated by reference, discloses the use of multiple lamps in combination with multiple concave reflectors to focus the image of a first lamp's arc onto another lamp's arc in an off-axis cascaded configuration. FIGS. 1a and 1b illustrate the concept of using a cascading series of reflectors and sources aligned in an off-axis relationship for the coupling of light into a target as described in the above patent. The system comprises three main components: a plurality of sources S.sub.i, a plurality of reflectors M.sub.i, and at least one target I.
The plurality of sources S.sub.i are typically point sources of electromagnetic radiation, such as a high intensity arc lamp having an arc gap. However, any compact source of electromagnetic radiation with a small area of emission would be suitable.
The plurality of reflectors M.sub.i focus electromagnetic radiation from the source S.sub.i to at least one target I. In the '131 patent, the reflectors are mirrors having a concave surface facing the source and target. The reflective surfaces of the reflectors are either spherically, toroidally, or ellipsoidally shaped such that off-axis reflection can be achieved as is known in the art.
The target is a small object which is to be provided with illumination having the highest density of electromagnetic radiation possible, such as the end of a single core optical fiber or other optical lightguides.
Although a non-cascaded off-axis system produces minimal magnification which approximates a 1:1 magnification when the source to target distance is small, for the cascaded configuration disclosed in the '131 patent, the small amount of magnification created by the off-axis system will multiply and propagate as more lamps and reflectors are added to a cascade. Thus, significant magnification is often experienced at the target spot, corresponding to a decrease in flux density, and the amount of light coupled to the target is decreased. Because of the aforementioned and other drawbacks inherent in the prior art, there remains a need for an improved coupling system in which sources can be cascaded with a 1:1 magnification such that the efficiency of the cascade is not decreased as more lamps are added.